To the extent that the shipping containers are likely to be of one of the larger configurations in common use, I would not expect significant effects from "resonance" of the boxes. The typical kinds actually are fairly resistant to transmission of sounds through the walls, as would be required for activation/compression of the large internal air spaces.

(A few cases in which the screams of the illegal migrants trapped inside one or another of the common ones were not heard offer an "experimental verification(?)" of this.)

The main concern, to the extent that the containers form a barrier would be with simple transmission through them, and in the near-field surrounding area. The critical factor affecting transmission is that the acoustic impedence of the container walls is significantly different than the impedence of the free air surrounding them.

In the simple case, all the sound that is not transmitted through the sidewall where the sound impinges must be reflected off the wall, with very little dissipation. With the barrier construction implied, sound transmitted through the containers should be reasonably attenuated, but even low efficiency reflection does nothing to reduce the total amount of sound energy, and "spillover" in the acoustic near-field cannot be expected to be reduced much.

In order to make the containers sound absorbent the energy must pass into the barrier in a way that dissipates much of the energy. This can be accomplished by drilling holes through the surface on which the sound impinges, since even "acoustic flow" through a small hole dissipates (essentially as waste heating) a large percentage of the impinging sound.

Holes of about 1/4 to 1/2 inch in diameter spaced around an inch apart, covering the entire side of the barrier facing the noise source should do quite nicely. Ideally, the sizes should vary randomly between the limits chosen. The suggested hole sizes are significantly larger than is commonly used for the internal noise reduction shrouds used to quiet modern jet engines (quite effectively); but the frequencies requiring absorbtion in the engines include very high frequency "whine" components not likely to be present in output of which the festival amplifiers are capable, so some increase in hole diameters is appropriate. The holes must cover very nearly the entire surface of the barrier facing the impending noise.

Since typical shipping containers are made from "hot rolled" steel or "tempered alumin(i)um," both of which resist drilling, somewhat larger holes than are optimum for maximum sound absorbtion are a trade-off with number of drill bits consumed, since it will require "a whole bunch of holes" drilled through very tough material.

It may be noted that the owners of the containers may have some objection to using this method, as it's doubtful that the festival management obtained permanent (legal) possession of them, so it would be important to ascertain whether the festival bears responsibility for their intact return, rather than those who drill the holes.

Regardless of how effective the barriers are or can be modified to be, the "open topped" venue and the placement of speakers that I visualize from the scant descriptions make it unlikely that the presence or absence of the barriers will have any significant effect on the actual sound levels outside the park, although effective absorbtion offers slightly better odds of "not making it worse" than is the case with reflective walls, which could just make a louder echo.

John